Find yourself a dead on side view photograph of the El Camino and print it out. You know the wheelbase on the real car is 116 inches. The wheelbase on your model is going to be 1/25 of 116 or 4.64 inches. (116 / 25 = 4.64)

Measure the wheelbase on your photograph.

Let's say your wheelbase on your photo is 5.50 inches. Divide 4.64 by 5.5. This will give you a scale factor of 0.843636.

Write 0.843636 on your photo. Now ANY measurement you take off the photo you multiply it by 0.843636. That will give you the measurement it should be on the model.